This invention relates to a vented cell structure and, more particularly, to a vented cell structure that permits the rapid dissipation of pressure differentials between the cell structure and the surrounding environment.
Cellular or honeycomb cores (hereinafter, honeycomb) are utilized in various applications to provide lightweight, but relatively strong structural support. For example, honeycomb is used, typically in the body structure, of automobiles, tractor-trailers, and other land vehicles, in addition to the body structure of aircraft, spacecraft, and other air vehicles. In particular, honeycomb is used in aircraft flaps, wing-to-body fairings, nacelles, radomes, doors, floors, ceilings, stow bins and walls, and in the internal sandwich structure in cryogenic tanks. For instance, honeycomb structures are used in the wing forward, leading edge wing panels, and from the tip of the wing to the fuselage in the Boeing 747 aircraft.
Honeycomb is made of multiple cell walls that define multiple cells. The cell walls are typically made of metallic and/or nonmetallic material. Many types of honeycomb are made of cellulose material. One example of a nonmetallic material used to make honeycomb is NOMEX(copyright), commercially available from E.I. du Pont de Nemours and Company, Inc., which is formed of resin impregnated fibers that provide not only the necessary lightweight and high-strength characteristics, but also thermal protection. In addition, the cell walls of the honeycomb may be arranged in various configurations include, hexagonal, overexpanded, and flexible core, each having different cell geometry similar to a hexagon shape.
Honeycomb panels typically include load carrying or structural layers on opposite sides of the honeycomb cellular structure. The structural layers are usually made of structural fiber reinforced composite material or a metallic material. The honeycomb is typically attached to the structural layers with an adhesive material applied between the edges of the cell walls of the honeycomb and the structural layer. The resulting honeycomb panel is then capable of withstanding significant in-plane, bending and shear loads.
When the honeycomb panel is subject to repeated changes in temperature and pressure, however, the advantages of the honeycomb panel may be reduced.
For example, in the aircraft industry, the honeycomb panel in the wings of an aircraft contains air, and the pressure and temperature of the air changes to be in equilibrium with the outside atmosphere. Thus, the air in the honeycomb panel experiences repeated changes in pressure and temperature between the time when the aircraft is in flight and the time when the aircraft is on the ground. When the aircraft is on the ground, the cells of the honeycomb are in equilibrium with the temperature and pressure of the air on the ground. When the altitude of the aircraft increases, however, the pressure and temperature of the atmosphere decreases. The pressure and temperature of the air in the wing and in the honeycomb panel also decrease and eventually become substantially equal to the lower pressure and temperature of the atmosphere at the increased altitude. In this regard, air generally gradually leaks from the honeycomb panel, such as though the cell walls and structural layers to equalize the pressure. When the aircraft returns to the ground, the pressure and temperature of the atmosphere are greater than the pressure and temperature of the air in the honeycomb panel, such that the air in the wing and honeycomb panel must increase in pressure and temperature in order to equalize with the outside atmosphere. For example, air again generally leaks through the honeycomb panel to increase the internal pressure. The increase in air pressure and temperature causes water to condense within and on the outside of the honeycomb panel. The water on the outside of the honeycomb panel may eventually migrate through the structural layers, which adds to the moisture inside the honeycomb. The next time the aircraft is in flight and subjected to an atmosphere at a lower pressure and temperature, at least some of the water may evaporate, but over time, some water will get trapped in the honeycomb material because it may take longer for the water to evaporate than the time that the aircraft is in the lower temperature and pressure atmosphere. The cell walls of the honeycomb material will eventually absorb the water, which makes evaporation of the water an even more lengthy process. The water in the honeycomb material then makes the honeycomb panel heavier and causes the honeycomb panel to lose its structural integrity.
Honeycomb material in a honeycomb panel that has absorbed water, therefore, may occasionally be replaced with new honeycomb material. In the aircraft industry, replacing the honeycomb material consists of several hours of labor because the segment of the aircraft with the damaged honeycomb material must be taken apart, the damaged honeycomb material removed, new honeycomb material installed, and the segment of the aircraft rebuilt. Thus, the aircraft must be taken out of service and significant amounts of labor and time must be expended to replace the honeycomb panel, which is costly.
Attempts to eliminate the moisture collection in honeycomb panels have focused on eliminating the pressure and temperature differential between the interior of the honeycomb panel and the exterior of the panel. In order to eliminate the pressure and temperature differential, the entrapped air characteristic of honeycomb panels must be mitigated to allow the air within the honeycomb panel to equalize with the air outside the honeycomb panel immediately instead of slowly, which causes the pressure differential that promotes moisture accumulation. Without the pressure and temperature differential caused by air at one temperature and pressure being trapped in the honeycomb panels and air at another temperature and pressure outside of the panel, moisture does not get trapped in the honeycomb panel, which extends the life of the honeycomb material because the honeycomb material will not have to be replaced due to moisture problems.
Existing techniques for allowing fluids to flow in and out of the honeycomb panel without obstruction consist of perforating the cell walls of the honeycomb material. For instance, honeycomb material with pre-perforated cell walls is available, such as that commercially available from Hexcel, Inc. The pre-perforated honeycomb material, however, is expensive. Another technique requires drilling through standard, non-perforated honeycomb material with a long drill bit from a side of the sandwich panel in order to puncture each cell wall. This technique permits the flow of fluid in and out of the honeycomb, but the drilling operation is time-consuming and expensive. In addition, the drilled holes in the honeycomb cells may reduce the structural integrity of the honeycomb panel.
In addition, the honeycomb panel may be formed of staggered layers of honeycomb materials in which two or more honeycomb layers are positioned side-by-side in an offset manner such that the cellular structure of the honeycomb panels is somewhat staggered. This honeycomb layered structure may also include perforated sheet(s) of polyester, polyvinylchoride, polyethylene, polypropylene, and polystyrene between adjacent honeycomb layers. The structural layers on the outer surface of the honeycomb panel also may define holes, at least some of which are aligned with the cells of the internal honeycomb material. This honeycomb layered structure configuration, however, does not permit the unobstructed flow of fluid between the cells of the staggered honeycomb because the perforated sheets between the cells may continue to trap at least some air within the honeycomb cells. As such, the temperature and pressure differentials may not dissipate as quickly as necessary to prevent moisture from forming.
Therefore, a need exists for a vented honeycomb structure that effectively and efficiently allows fluids to flow in and out of the honeycomb panel without any obstructions that may trap gases and/or liquid in the honeycomb cells. The vented honeycomb structure should be inexpensive to fabricate, but also continue to provide the necessary strength to withstand the in-plane, bending, and shear loads that the honeycomb panel will experience in a specific application.
The vented cell structure and fabrication method of the present invention provide a multilayer, vented honeycomb structure without obstructions in or around the honeycomb cells that may trap gases and/or liquids, such that fluids may freely flow in and out of the honeycomb panel. Thus, because gases and/or liquids cannot be trapped in the honeycomb cells, the honeycomb cells do not suffer from the detrimental effects that occur when moisture is trapped within the honeycomb material that may lead to replacement of the honeycomb material. For example, in the aircraft industry, the vented cell structure of the present invention decreases the repair/replacement costs associated with an aircraft because it prevents having to take the aircraft out of service in order to replace the honeycomb panels due to moisture damage. In addition, the vented cell structure of the present invention is inexpensive to fabricate, but provides the necessary strength to withstand in-plane, bending and shear loads that the honeycomb panels experiences in specific applications.
The vented cell structure of the present invention includes adjacent and connected first and second honeycomb layers having a plurality of first cell walls and second cell walls, respectively. The first and second cell walls, in turn, define a plurality of first cells and second cells, respectively. The first and second honeycomb layers are positioned such that the first and second cells are misaligned. For instance, the first honeycomb layer may be rotated relative to the second honeycomb layer. In a specific embodiment of the vented cell structure of the present invention, the first honeycomb layer is rotated ninety-degrees relative to the second honeycomb layer. Furthermore, after positioning the first and second honeycomb layers appropriately, pressure may be applied to the first and/or second honeycomb layer such that the edges of at least one of the first and second cell walls partially cut into one another, thereby fixing the first and second honeycomb layers in position relative to one another.
Various embodiments of the vented cell structure of the present invention include the first cells having the same shape and size or a different shape and size than the second cells. The first and/or second honeycomb layers may include a hexagonal, overexpanded and/or flexible honeycomb cell configuration. In addition, the first and/or second honeycomb layer may be made of a metallic material or a composite material.
The first and second honeycomb layers are connected only at intersections of the first and second cell walls. The intersections are the points where the first and second cell walls are in contact, whether one of the first and second cell walls is partially cut into the other or not. As such, the fluid can flow without obstruction between the first and second cells. To connect the first and second honeycomb layers, adhesive may be applied between the first and second honeycomb layers on only the first and second cell walls. Thus, the adhesive does not create any obstruction between the first and second cells, and fluid may freely flow between the first and second cells. The adhesive may be reticulated unsupported film adhesive. To apply the reticulated adhesive, unsupported reticulating film adhesive may be laid on the first and second honeycomb layers, and the adhesive may then be heated to draw the adhesive to the first and second cell walls. The resulting adhesive is therefore located on only the first and second cell walls, and does not create any obstruction between the first and second cell walls.
The first and second honeycomb layers may also be bonded to one another after properly positioning the first and second honeycomb layers relative to each other as described above. Bonding the first and second honeycomb layers includes applying pressure to the first and/or second honeycomb layer in a direction that is normal to a major surface of the first and/or second honeycomb layer, then heating the first and second honeycomb layers. Pressure may be applied to the first and second honeycomb layers by encasing the first and second honeycomb layers, and removing the air from the encasement. Encasing the first and second honeycomb layers may include placing a film, such as a nylon film, on an outward facing surface of the first and/or second honeycomb layer. When the air is removed from the encasement, the film may be in contact with the outward facing side(s) of the first and/or second honeycomb layer, such that the film applies pressure to the first and/or second honeycomb layer. Thus, the pressure applied to the first and/or second honeycomb layers holds the honeycomb layers in position as they are heated in order to bond the honeycomb layers to one another.
The vented cell structure also includes a structural layer that may be applied on opposite sides of the first and second honeycomb layers. The structural layers may be bonded to the honeycomb layers at the same time the honeycomb layers are bonded together or the structural layers may be adhered to the already bonded honeycomb layers. In a particular embodiment of the vented cell structure of the present invention, the structural layers are made of structural fiber reinforced composite material, such as fiberglass/epoxy or carbon fiber/epoxy, or a metallic material, such as aluminum or titanium. The structural layers also define at least one opening aligned with a first cell and/or a second cell. At least one opening may be defined in the structural layers prior to or subsequent to applying the structural layers on opposite sides of the first and second honeycomb layers. Thus, the resulting vented cell structure allows fluid to flow between the first and second cells without obstruction, and the fluid can also freely flow into and out of the first and second cells through the opening(s) in the structural layers, thereby reducing the possibility of moisture accumulating within the honeycomb material and causing damage.